(CNN) -- A spokesman for Illinois Sen. Roland Burris resigned Thursday in the wake of new disclosures about requests that Burris raise money for disgraced former Gov. Rod Blagojevich while seeking a U.S. Senate appointment.

Sen. Roland Burris, who is under fire to resign, arrives at a Chicago City Club luncheon to speak Wednesday.

"I initially began helping the senator on a temporary basis because he is a long-term friend who I served several years when he was [Illinois] attorney general," Jason Erkes said in a written statement to CNN.

"It is now time for me to get back to focusing on ... my newly formed strategic communications business."

The statement did not comment on the new disclosures.

A defiant Sen. Burris insisted Wednesday that he was innocent of any wrongdoing in his appointment to President Barack Obama's former Senate seat, as calls for his resignation intensified.

"I've done nothing wrong, and I have absolutely nothing to hide," Burris said in a speech to the City Club of Chicago. "You know the real Roland ... Stop the rush to judgment."

Burris told reporters earlier in the week that he informed Blagojevich's brother Robert in November that it would be inappropriate to raise money because he was interested in a Senate seat.

During his testimony under oath in Blagojevich's impeachment trial, however, Burris failed to mention any such conversations when asked about contacts with the governor's office. He later acknowledged that his testimony might be incomplete and filed an affidavit correcting it.

Burris said Wednesday that he did not hide his desire to be appointed to the Senate, but never had a conversation about a possible appointment with anyone close to Blagojevich other than the former governor's attorney.

"The governor's brother reached out to [me] to do fundraising," Burris conceded. "But I did not give one single dollar to the governor."

Illinois' senior senator, Dick Durbin, a member of the Senate Democratic leadership, told reporters during a congressional trip to Europe that "the Ethics Committee of the Senate is undertaking an investigation."

"Everyday there are more and more revelations about contacts with Blagojevich advisers, efforts at fundraising and omissions from his list of lobbying clients," Durbin later said in a written release. "This was not the full disclosure under oath that we asked for.

"These news reports and the public statements by Roland Burris himself are troubling and raise serious questions which need to be looked at very carefully."